docs: overhaul README with system architecture diagram and roadmap

[EmeditWeb]

Summary

Documentation-only overhaul of the repository README so StepFi-Web reads like the rest of the protocol repos. Adds a committed system-architecture diagram (public/architecture.svg) that highlights StepFi-Web's role in the whole system, a scripts reference, a CI/CD section, and a roadmap table. Also adds the MIT LICENSE file the README badge was already pointing at.

This repo is for the React web app only

No product code changed — this is docs + a static SVG asset + the license file.

• Changes are documentation only (README.md, LICENSE, public/architecture.svg)
• No Rust, Soroban, or contract code added
• No React Native or Expo-specific code added
• No hardcoded hex colors in app code (the SVG asset is a diagram, not UI)
• No new icons added
• No API calls added

Type of change

• Documentation

What changed

• Embedded public/architecture.svg — the StepFi system diagram with the Web box highlighted as "this repo".
• Added a Scripts table (dev / build / preview / lint / test / test:coverage / test:a11y).
• Added a CI/CD section (required ci.yml gate; Netlify + Vercel deploys).
• Added a Roadmap table reflecting current sponsor / vendor / mentor feature status.
• Added the MIT LICENSE file (the README badge already referenced ./LICENSE).

Testing

• Docs-only change; no code paths affected. Existing ci.yml (lint · test · build) remains the gate.